This invention relates to hydraulic boosters for vehicle hydraulic systems, especially braking systems, of the kind in which a boost piston working in a bore in a body is adapted to apply an output force to an output member, for example for operating an hydraulic master cylinder, in response to a pressure applied to a power chamber under the control of a control valve, in turn responsive to an input force. Hydraulic boosters of this kind are hereinafter referred to as hydraulic boosters "of the kind set forth".
In hydraulic boosters of the kind set forth difficulties are often experienced in maintaining concentricity between components in order to ensure that seals provided between adjacent chambers are effective, particularly when the booster is of the closed-center type and is disposed in a retracted "inoperative" position.
When the booster is of the closed-center type in which fluid is metered across an orifice by operation of the control valve, a constant flow of fluid is required during operation. This can be a drain on the supply, particularly during a long operating cycle, thereby making it unsuitable for some occasions.